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Two whistleblowers offer a rare window into the root causes of the subprime mortgage meltdown. Eileen Foster, a former senior executive at Countrywide Financial, and Richard Bowen, a former vice president at Citigroup, tell Steve Kroft the companies ignored their repeated warnings about defective, even fraudulent mortgages. The result, experts say, was a cascading wave of mortgage defaults for which virtually no high-ranking Wall Street executives have been prosecuted.

The following is a script of "Prosecuting Wall Street" which aired on Dec. 4, 2011. Steve Kroft is correspondent, James Jacoby, producer.

It's been three years since the financial crisis crippled the American economy, and much to the consternation of the general public and the demonstrators on Wall Street, there has not been a single prosecution of a high-ranking Wall Street executive or major financial firm even though fraud and financial misrepresentations played a significant role in the meltdown. We wanted to know why, so nine months ago we began looking for cases that might have prosecutorial merit. Tonight you'll hear about two of them. We begin with a woman named Eileen Foster, a senior executive at Countrywide Financial, one of the epicenters of the crisis.

Steve Kroft: Do you believe that there are people at Countrywide who belong behind bars?

Eileen Foster: Yes.

Kroft: Do you want to give me their names?

Foster: No.

Kroft: Would you give their names to a grand jury if you were asked?

Foster: Yes.

But Eileen Foster has never been asked - and never spoken to the Justice Department - even though she was Countrywide's executive vice president in charge of fraud investigations. At the height of the housing bubble, Countrywide Financial was the largest mortgage lender in the country and the loans it made were among the worst, a third ending up in foreclosure or default, many because of mortgage fraud.

It was Foster's job to monitor and investigate allegations of fraud against Countrywide employees and make sure they were reported to the board of directors and the Treasury Department.

Kroft: How much fraud was there at Countrywide?

Foster: From what I saw, the types of things I saw, it was-- it appeared systemic. It, it wasn't just one individual or two or three individuals, it was branches of individuals, it was regions of individuals.

Kroft: What you seem to be saying was it was just a way of doing business?

Foster: Yes.

In 2007, Foster sent a team to the Boston area to search several branch offices of Countrywide's subprime division - the division that lent to borrowers with poor credit. The investigators rummaged through the office's recycling bins and found evidence that Countrywide loan officers were forging and manipulating borrowers' income and asset statements to help them get loans they weren't qualified for and couldn't afford.

Foster: All of the-- the recycle bins, whenever we looked through those they were full of, you know, signatures that had been cut off of one document and put onto another and then photocopied, you know, or faxed and then the-- you know, the creation thrown-- thrown in the recycle bin.

Kroft: And the incentive for the people at Countrywide to do that was what?